The wiper arm carries, via the blade carrier, an articulated wiper blade which sweeps over the glass (or swept) surface, which may for example be a windshield of the motor vehicle. In order to obtain correct wiping of the glass surface, it is necessary that the wiping strip of the blade should be applied on the swept surface with a substantial pressing or wiping force.
To this end, at least one spring is usually provided, for example a helical tension spring which is disposed between the drive head and the remainder of the wiper arm, and which in practice applies a substantially constant nominal wiping torque to the wiper arm about its pivot axis, in such a way as to cause the wiping strip to be forced firmly against the glass. The design of such an arrangement is such that the wiping strip is forced against the swept surface not only during the wiping movement itself but also in the rest or parking position of the wiping apparatus. Since the nominal wiping force applied to the wiping strip is permanent, even while the apparatus is at rest, it is found that the wiping strip retains a residual shape which results from the distortion of its profile against the swept surface. This results in the strip eventually ceasing to wipe the swept surface efficiently enough. It is therefore desirable to be able to reduce the nominal wiping force applied to the wiping strip while the apparatus is parked.
In addition, the quality of wiping of the swept surface depends in operation on the magnitude of the wiping force which is applied to the strip, and this is especially dependent on the speed of movement of the vehicle and the frequency of the wiping movement of the apparatus itself.
It is also known to increase the wiping force by means of aerodynamic attachments which are mounted on the wiper arm. However, these arrangements do not enable the value of the wiping force to be controlled with any precision. In particular, they do not enable the force to be adjusted in a controlled manner in accordance with an operating parameter of the vehicle.
In order to overcome the drawbacks mentioned above, it has already been proposed to provide a screen wiping apparatus of the general type mentioned above, in which the means which are arranged between the drive head and the wiper arm in order to apply a wiping torque to the latter comprise at least one spring, together with a device for applying a complementary couple to the arm. The spring applies a substantially constant nominal wiping torque to the wiper arm, while the algebraic value of the complementary torque applied by the said device can, in particular, be adjusted in such a way that the value of the wiping torque can be increased or reduced as a function of at least one operating parameter of the vehicle.
The complementary torque-applying device comprises, for example, an actuator which is associated with the drive head, and which has an output ram coupled to the wiper arm.